MEMS (Micro Electro Mechanical Systems) optical switches have received a great deal of attention as a hardware technology to implement large-scale optical switches. The most characteristic component of a MEMS optical switch is a MEMS mirror array. The MEMS mirror array includes a plurality of MEMS mirror devices (to be referred to as mirror devices hereinafter) arrayed two-dimensionally. A conventional mirror device (see, e.g., Japanese Patent No. 3579015) will be described below.
As shown in FIGS. 107 and 108, an insulating layer 8002 made of a silicon oxide film is formed on a lower substrate 8001 of single-crystal silicon. Four driving electrodes 8003-1 to 8003-4 are provided on the insulating layer 8002 at the center of the substrate 8001. Supports 8004 of single-crystal silicon are provided on both sides of the upper surface of the tower substrate 8001.
An upper substrate 8101 has an annular gimbal 8102 inside. A mirror 8103 is provided inside the gimbal 8102. For example, a Ti/Pt/Au layer (not shown) with a three-layered structure is formed on the upper surface of the mirror 8103. Torsion springs 8104 connect the upper substrate 8101 to the gimbal 8102 at two 180° opposite points. Similarly, torsion springs 8105 connect the gimbal 8102 to the mirror 8103 at two 180° opposite points. The X-axis passing through the pair of torsion springs 8104 and the Y-axis passing through the pair of torsion springs 8105 intersect at a right angle. As a result, the mirror 8103 can pivot around the X- and Y-axes each serving as a pivot axis. The upper substrate 8101, gimbal 8102, mirror 8103, and torsion springs 8104 and 8105 are integrally made of single-crystal silicon.
The structure of the lower substrate 8001 and the structure of the upper substrate 8101 shown in FIGS. 107 and 108 are separately manufactured. The upper substrate 8101 is soldered to the supports 8004 so that the upper substrate 8101 bonds to the lower substrate 8001. In this mirror device, the mirror 8103 is grounded. A positive voltage is applied to the driving electrodes 8003-1 to 8003-4 to generate an asymmetrical potential difference between the driving electrodes 8003-1 to 8003-4. An electrostatic force attracts the mirror 8103 and causes it to pivot in an arbitrary direction.
The design of the mirror device whose driving electrodes 8003-1 to 8003-4 generate an electrostatic force to drive the mirror 8103 is based on the fact that the electrostatic force is proportional to the second power of size, i.e., the area, unlike the gravity or inertial force that is proportional to the third power of size, i.e., the volume. For the usual order of centimeters, the presence of an electrostatic force is noticeable only in frictional electricity with a high voltage of several thousand V or more. As the size reduces, an inertial force abruptly becomes small in proportional to the third power of size. However, the electrostatic force decreases in proportional to the second power of size. Hence, the electrostatic force can lift or move an object even at a low voltage of several V to several ten V in a microscopic world. In the mirror device shown in FIGS. 107 and 108, the diameter of the mirror 8103 is, e.g., about 500 μm. The distance between the mirror 8103 and the driving electrodes 8003-1 to 8003-4 is, e.g., about 90 μm.
Frictional electricity causes spark discharge due to its high voltage. In a small mirror device, however, no avalanche discharge with spark occurs with the same electric field strength. This is because even when the electric field is strong, particles (particles ionized due to some reason, e.g., ions in air that are ionized by cosmic rays or natural radiation) accelerated by it cannot acquire energy so high as to ionize other neutral particles collided with them because of the short distance between the mirror 8103 and the driving electrodes 8003-1 to 8003-4. The electrostatic force is proportional to the electric field strength between the electrodes (in the mirror device, between the mirror 8103 and the driving electrodes 8003-1 to 8003-4). Hence, if the interelectrode distance is long, it is necessary to give a large voltage difference between the electrodes. However, the large voltage difference applied between the electrodes may cause discharge, as described above. Even with the same electric field strength, the voltage difference applied between the electrodes can decrease in proportional to the interelectrode distance in the small mirror device. Since the above-described factor prevents discharge, a stable driving force is available. The reasons that mainly make the electrostatic force effective as a driving force in the mirror device have been described above. Use of the electrostatic force allows to control the driving force by the voltage applied to the driving electrodes 8003-1 to 8003-4. Since control by an electronic circuit is easy, and any steadily flowing current does not exist, power consumption greatly decreases.